And there's always the possibility of a factory screw up. What if the firewall stampings on 2 cars got mixed up on the assembly line? Could be easy enough for a worker to get distracted and forget which number went on which shell.
As Steve noted, what other numbers are on the car and do they match?
I was a service writer at a Ford dealer in the 1980's and we once received a Thunderbird from the factory with an incorrect VIN. It was a major project to correct. The zone rep was there with the replacement VIN tag, the dealership service manager was there, a glass guy was there (to pop and reinstall the windshield), a tech to do the work, a couple of witnesses, and a photographer to record the event.
